Process for the purification of nitrogen-contaminated alkanedioic acids and esters



United States Patent Ofiice PROCESS FDR THE PURIFECATIDN F NITRO- GEN-CONTAMINATED ALKANEDIOIC ACIDS AND ESTERS George P. Brown, Jr., West Deer Township, Allegheny County, Harry J. Elder, Tarenturn, and Norman W. Franke, Penn Hills, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware No Drawing. Filed July 1, 1958, Ser. No. 745,804 3 Claims. (Cl. 260-485) This invention rel-ates to an improved process for re fining dibasic acids and dibasic acid esters, and more particularly to a process for refining dibasic acids and dibasic acid esters containing nitrogen impurities.

Dibasic acids can be obtained by oxidizing hydrocarbons, aliphatic, naphthenic or aromatic, or air-oxidized derivatives thereof, with nitric acid. Unfortunately such dibasic acids, as Well as the esters produced therefrom, have a high nitrogen content, which is believed to be due to the presence of polyfunctional compounds. containing carboxyl and nitro, nitrate or other nitrogen-containing groups. The amount of nitrogen present can be substantial, from about 0.1 to about 8 percent by weight of the dibasic acid or the ester derived therefrom. Because of the adverse effects which such nitrogen-containing compounds have on dibasic acids and dibasic acid esters when they are employed in commercial practice, it is obvious that a great need exists to purify such acids and esters and render them commercially acceptable.

We have found that dibasic acids and esters prepared therefrom can be treated and a product having a low nitrogen content can be obtained by a process which comprises subjecting such acids or esters to a mild hydrogenation treatment and thereafter recovering the nitrogen-containing compounds therefrom.

The dibasic acids which can be treated in accordance with the present invention are those which have been prepared as a result of the nitric acid oxidation of hydrocarbons. Such dibasic acids can be prepared, for example, by oxidizing a hydrocarbon, aliphatic, naphthenic or aromatic, or air-oxidized derivatives thereof, with nitric acid having a concentration of about 10 to about 100 percent, preferably about 50 to about 95 percent at a temperature of about 50 to about 150 C., preferably about 75 to about 130 C., and a pressure of about atmospheric to about 1500 pounds per square inch, preferably about 100 to 1000 pounds per square inch, for about minutes to about 60 hours, preferably about 15 minutes to about 2 hours. The dibasic acid so obtained can then be treated, for example, by distillation at a temperature of about 30 to about 75 C., and a pressure of about 5 to about 100 millimeters of mercury, for the purpose of removing unreacted nitric acid therefrom and obtaining a product predominating in dibasic acids having 3 to 12 carbon atoms.

This process is particularly effective in the purification of dibasic acids obtained in accordance with the process disclosed and claimed in US. Patent No. 2,771,482, dated November 20, 1956 to George P. Brown, J11, Edgar I. Crowley and Norman W. Franke and assigned to the assignee of this invention. Briefly in the process of the patent, a saturated aliphatic hydrocarbon is initially subjected to oxidation with air or other oxygen-containing gas, that is, a gas containing free oxygen, for a time sufiici-ent to obtain a product having a saponification number above about 100, and preferably between about 150 and 450, and the product obtained is thereafter subjected to further oxidation at an elevated temperature with nitric acid having a concentration above about 50, preferably above about 60 percent, for a time sufficient Patented Oct. 31, 1961 to obtain substantial amounts of dibasic acids having 3 to 12 carbon atoms, such as succinic, glutaric, adipic, etc.

Alcohols which are employed in preparing the dibasic acid esters, monoesters as well as diesters, which are to be treated in accordance with the process of this invention are aliphatic monohydric alcohols containing one or two carbon atoms per molecule.

The conditions under which the esterification reaction resulting in the dibasic acid esters which are treated in the present process are carried out are conventional and need not be described in great detail. Thus, the esterification is satisfactorily carried out by refluxing the dibasic acid with an excess of the monohydric alcohol, for example, a 3:1 mol ratio of alcohol to acid, preferably in the presence of a small amount of a suitable catalyst, e.g., sulfuric acid, dry hydrogen chloride, p-toluene sulfonic acid, or the like, for a period of about 3 to about 10 hours, for example, 5 hours. The time of reaction normally decreases with increased temperature of the reaction mixture. Although esterification can proceed to some degree at temperatures as low as room temperature, elevated temperatures on the order of about 60 to about 240 C. are preferably employed.

Examples of dibasic acid esters which can be purified in accordance with the present process are dimethyl succinate, dimethyl glutarate, dimethyl adipate, diethyl succinate, diethyl glutarate, diethyl adipate, etc. The dibasic acids, and the dibasic acid esters produced therefrom, which are purified in the present process have many uses but are of particular utility as intermediates in the manufacture of plastics, plasticizers, lubricants, etc.

In purifying the dibasic acids and the dibasic acid esters of the defined impurities, the first step involves hydrogenating the same under fairly mild conditions. Any suitable hydrogenation catalyst, for example, cobalt, nickel, platinum, palladium or other noble metals can be employed. The concentration of catalyst is not critical and can be from about one-tenth to about 50 percent by weight, based on the charge, but preferably is within the range of about one to about ten percent by weight. The pressure similarly is not critical and can be about atmospheric to about 5000 pounds per square inch, but preferably is in the range of about to about 2000 pounds per square inch. In order to obtain a high recovery of liquid and reduce loss of compounds in the form of gas, the temperature should preferably be kept below 300 C. but preferably below 250 C. As a lower limit, a temperature of about 50 C. can be employed, although a lower limit of about 100 C. is preferred. The amount of hydrogen necessary for the reaction is dependent of course upon the other variables involved, but preferably should be. at least about /2 cubic foot per pound of charge, preferably about one to about 20 standard cubic feet per hour per pound of charge being treated per hour. The duration of the hydrogenation treatment can be about hour to about 20 hours and preferably about hour to about 5 hours. Conveniently the acid or ester charge is subjected to hydrogenation while in admixture with an organic solvent such as methyl alcohol, dioxane, acetic acid, acetic anhydride, other alcohols and esters.

Following the hydrogenation stage, the hydrogenated product obtained is treated to remove the resulting nitro gen compounds present in admixture with the dibasic acid or dibasic acid esters. In the event the initial charge is a dibasic acid ester, the hydrogenated product obtained is subjected to a washing step with an aqueous acid solution, preferably an aqueous mineral acid solution such as hydrochloric acid, sulfuric acid, etc. The concentration of acid in the wash solution is not critical and can vary from about one ot about 20 percent by weight. The washing can be effected in a single or multi-stage extraction or, if desired, can be a continuous countercurrent extraction. The amount of aqueous solution necessary in the washing step is not critical and can vary from about 10 to about 300 percent by weight based on the hydrogenated product being treated. In order to avoid hydrolysis of esters the extraction temperature is maintained conveniently low, preferably from about to about 50 C. The pressure similarly is kept low and can vary from about to about 100 pounds per square inch absolute. The duration of the washing step can be about one to about 100 minutes.

While the hydrogenation step described above followed by washing is effective in removing a substantial portion of the impure nitrogen-containing compounds present in the dibasic acid esters, in the preferred embodiment of the invention the product at this point is usually subjected to further treatment with an adsorbent material such as carbon, in order to remove substantially all of the nitrogen=containing compounds remaining in admixture With the esters. In the case where the adsorbent is carbon, any effective method can be employed which will bring intimate contact between the treated ester and the carbon. Preferably the ester is passed through a column packed with carbon having a particle size of about 5 to about 100 mesh, at a temperature of about 0 to about 100 C. and a pressure of about 5 to about 100 pounds per square inch absolute. The amount of carbon required relative to the charge is about to about 200 percent by weight.

In the event the product which has been subjected to hydrogenation is a dibasic acid, the hydrogenated product is not subjected to a washing step, since some of the dibasie acids present are soluble in an aqueous solution, but is subjected to treatment with an adsorbent material such as carbon in the manner described above.

This invention can further be illustrated by the following representative examples.

xample 1 2876 grams of a deoiled foo-ts oil was oxidized with air at atmospheric pressure and a temperature of 320 F. over a period of 10%. hours to obtain an oxidized product having a saponification number of 220. This was repeated several times and the products mixed. During the oxidation air was employed at a rate of 2 /2 liters of air per minute per kilogram of foots oil. 4231 grams of the air-oxidized mixture obtained was further oxidized with 13,000 grams of 65 percent nitric acid at 130 C. and 1000 pounds per square inch. The contact time was minutes. A mixture of C to C dibasic acids Was obtained having a saponification number of about 560. 100 grams of the mixture of dibasic acids was esterified with a total of 700 grams of methyl alcohol in the presence of 2 milliliters of 96 percent sulfuric acid as catalyst at atmospheric pressure and 81 C. over a period of one hour.

40 grams of the methyl ester blend prepared above containing 0.478 percent by weight of nitrogen was dissolved in 230 cc. of methyl alcohol and the resulting mixture was hydrogenated in the presence of 10 percent by weight of Raney nickel, based on the original charge, at a temperature of 200 C. and a pressure 1000 pounds per square inch over a period of four hours. The amount of hydrogen employed was about 0.5 standard cubic feet. The hydrogenated product obtained was thereafter washed three times, each time with 25 cc. of a three percent aqueous hydrochloric acid solution at a temperature of C. and atmospheric pressure. The amount of nitrogen compounds present in the treated product was 0.208 percent by weight, which amounted to a 56 percent reduction over the original charge.

While the above example clearly illustrates the fact that a substantial reduction in nitrogen content in a dibasic acid ester can be obtained by hydrogenating the same and thereafter washing with an aqueous acid solution, the following example shows that in the preferred embodiment of the invention an additional treatment with an adsorbent material, such as carbon, further reduces the amount of nitrogen compounds present to an almost insignificant amount.

Example II grams of the methyl ester blend obtained as in Example I analyzing 0.47 8 percent by weight of nitrogen was dissolved in 200 cc. of methyl alcohol and the resulting mixture was hydrogenated in the presence of 10 percent by weight of Raney nickel, based on the methyl ester blend, at a temperature of C. and 1000 pounds per square inch gauge over a period of 5 /2 hours. The amount of hydrogen employed was about 0.9 standard cubic feet. The hydrogenated product obtained was thereafter washed three times, each time with 40 cc. of a 3 percent aqueous hydrochloric acid solution. The ester blend resulting from such treatment contained 0.191 percent by weight of nitrogen, amounting to a 60 percent reduction. In order further to purify the methyl ester blend obtained, 50.6 grams thereof was diluted with ten times its volume of normal pentane and the same was passed through a column five feet in height and 10 millimeters in diameter packed with carbon having an average particle size of 20 to 50 mesh at room temperature and atmospheric pressure. The methyl ester thus obtained had a nitrogen content of 0.028 percent. This amounts to a 94 percent reduction in nitrogen over that present in the original charge.

In each of the above examples, the product being purilied is a dibasic acid ester. In the Example III below, it can be seen that the procedure of this invention can also be employed in purifying a dibasic acid contaminated with nitrogen impurities.

Example III 40 grams of the dibasic acid mixture as prepared in Example I having a nitrogen content of 1.25 percent by weight was dissolved in 208 grams of dioxane and the mixture was hydrogenated by treating in the presence of a supported platinum catalyst at a temperature of 150 C. and a pressure of 1000 p.s.i.g. over a period of five hours. The amount of hydrogen employed was about 1.0 standard cubic foot. The amount of nitrogen at this point was about 1.06 percent by weight. The hydrogenated product obtained was thereafter passed through a column five feet in length and 10 millimeters in diameter packed with carbon having an average particle size of 20 to 50 mesh at 20 C. and atmospheric pressure. The nitrogen content of the treated product was 0.22 percent by weight. This amounts to a reduction in nitrogen of 82.3 percent over the original charge.

Obviously many modifications and variations of the invention, as hereinabove set forth, can be made without departing from the spirit and scope thereof and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A process for purifying a compound selected from the group consisting of alkanedioic acids and the methyl and ethyl alkanedioic acid esters derived therefrom contaminated with nitrogen-containing compounds, said acids having been obtained as a result of the nitric acid oxidation of a saturated aliphatic hydrocarbon, which comprises mildly hydrogenating said compound in the presence of about one-tenth to 50' percent by weight, based on the charge, of a hydrogenation catalyst selected from the group consisting of cobalt, nickel, platinum and palladium and about /2 to 20 standard cubic feet per hour of hydrogen per pound of charge being treated per hour at a pressure of about atmospheric to 2000 pounds per square inch gauge and a temperature of about 50 to 300 C. for about hour to 20 hours, washing said hydrogenated ester with an acidic solution at a temperature of about 0 to 50 C. and thereafter contacting a member of the group consisting of the hydrogenated acid and the washed material with carbon, whereby substantially all of said nitrogen-containing contaminants are removed from the charge being treated.

2. A process for purifying an alkanedioic acid contaminated with nitrogen-containing compounds, said acid having been obtained as a result of the nitric acid oxidation of a saturated aliphatic hydrocarbon, which comprises mildly hydrogenating said acid in the presence of about one-tenth to 50 percent by weight, based on the charge, of a hydrogenation catalyst selected from the group consisting of cobalt, nickel, platinum and palladium and about /2 to 20 standard cubic feet per hour of hydrogen per pound of charge being treated per hour at a pressure of about atmospheric to 2000 pounds per square inch gauge and a temperature of about 50 to 300 C. for about 4 hour to 20 hours and thereafter contacting said hydrogenated acid with carbon, whereby substantially all of said nitrogen-containing contaminants are removed from the charge being treated.

3. A process for purifying a compound selected from the group consisting of the methyl and ethyl esters of alkanedioic acids contaminated with nitrogen-containing compounds, said acids having been obtained as a result of the nitric acid oxidation of a saturated aliphatic hydrocarbon, which comprises mildly hydrogenating said compound in the presence of about one-tenth to 50 percent by weight, based on the charge, of a hydrogenation catalyst selected from the group consisting of cobalt, nickel, platinum and palladium and about /2 to 20 standard cubic feet per hour of hydrogen per pound of charge being treated per hour at a pressure of about atmospheric to 2000 pounds per square inch gauge and a temperature of about to 300 C. for about hour to 20 hours, washing said hydrogenated ester with an acidic solution at a temperature of about 0 to 50 C. and thereafter contacting the Washed material with carbon, whereby substantially all of said nitrogen-containing contaminants are removed from the charge being treated.

Hassler: Active Carbon, 1951, pp. 53-74 and 200- 209.

Lederer et a1.: Chromatography, 1953, pp. 20 and 182-184. 

1. A PROCESS FOR PURIFYING A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKANEDIOIC ACID AND THE METHYL AND ETHYL ALKANEDIOIC ACID ESTERS DERIVED THEREFROM CONTAMINATED WITH NITROGEN-CONTAINING COMPOUNDS, SAID ACIDS BEEN OBTAINED AS A RESULT OF THE NITRIC ACID OXIDATION OF A SATURATED ALIPHATIC HYDROCARBON, WHICH COMPRISES MILDLY HYDROGENATING SAID COMPOUND IN THE PRESENCE OF ABOUT ONE-TENTH TO 50 PERCENT BY WEIGHT, BASED ON THE CHARGE, OF A HYDROGENATION CATALYST SELECTED FROM THE GROUP CONSISTING OF COBALT, NICKEL, PLATINUM AND PALLADIUM AND ABOUT 1/2 TO 20 STANDARD CUBIC FEET PER HOUR OF HYDROGEN PER POUND OF CHARGE BEING TREATED PER HOUR AT A PRESSURE OF ABOUT ATMOSPHERIC TO 2000 POUNDS PER SQUARE INCH GAUGE AND A TEMPERATURE OF ABOUT 50* TO 300*C. FOR ABOUT 1/10 HOUR TO 20 HOURS, WASHING SAID HYDROGENATED ESTER WITH AN ACIDIC SOLUTION AT A TEMPERATURE OF ABOUT 0* TO 50*C. AND THEREAFTER CONTACTING A MEMBER OF THE GROUP CONSISTING OF THE HYDROGENATED ACID AND THE WASHED MATERIAL WITH CARBON, WHEREBY SUBSTANTIALLY ALL OF SAID NITROGEN-CONTAINING CONTAMINANTS ARE REMOVED FROM THE CHARGE BEING TREATED. 